A multi-axis photoelectric sensor includes an elongated projector and an elongated optical receiver having light emitting elements and light receiving elements in their casings, respectively. The light emitting elements and light receiving elements are arranged so as to be opposed one by one, whereby a detection area (light curtain) is formed by a plurality of optical axes. The multi-axis photoelectric sensor used for safety purpose is connected to a power supply circuit of a machine within a dangerous area and outputs a high-level signal when all the optical axes are in a non-shielded state, while stops the signal output when any optical axis is shielded. With this configuration, when some sort of object including a person enters the dangerous area, the machine within the dangerous area stops to thereby prevent occurrence of an accident.
However, when it is necessary to prevent the sensor output from stopping due to passage of an object such as a workpiece to be used in the dangerous area or an object to be conveyed by an automated conveyer unit, a muting function is used. In general, to implement this function, an object detection sensor (hereinafter, referred to as “muting sensor”) is disposed around the detection area of the multi-axis photoelectric sensor so as to allow an ON-state of the output from the multi-axis photoelectric sensor to be maintained even when the optical axis is shielded on a condition that a detection signal from the muting sensor and a non-shielded/shielded state of the detection area of the multi-axis photoelectric sensor change according to a previously registered sequence.
FIGS. 9 (1) to 9 (4) illustrate an example of the muting processing performed for a workpiece W passing through a conveyance path RD running toward the dangerous area. Reference numeral 1 denotes a projector of the multi-axis photoelectric sensor, and 2 denotes an optical receiver of the multi-axis photoelectric sensor. A pair of muting sensors A1, B1 are arranged on an upstream side (hereinafter, referred to as “safety side”) relative to the multi-axis photoelectric sensor and, similarly, a pair of muting sensors A2, B2 are arranged on a downstream side (hereinafter, referred to as “danger side”) relative to the multi-axis photoelectric sensor. Although transmission type photoelectric sensors are used as the muting sensors A1, B1, A2, B2 in this example, and projectors and optical receivers of respective muting sensors are alternately arranged, the type and installation method of the muting sensors are not limited to those described above. Further, a device other than the sensor having a function of detecting an object may be used in place of the muting sensors.
FIG. 9 (1) illustrates a state before the muting, in which the workpiece W is located on the upstream side relative to the muting sensors A1, B1. When the workpiece W advances, optical axes of the safety side muting sensors A1, B1 are shielded by the workpiece W in this order. In this example, as illustrated in FIG. 9 (2), when the safety side muting sensors A1, B1 are turned ON, the muting starts before the detection area of the multi-axis photoelectric sensor is shielded. As a result, even when the workpiece W further advances to shield the detection area, the ON-state of the output is maintained.
When the workpiece W enters the danger side, the safety side muting sensors A1, B1 are turned OFF in this order. Whereas, as illustrated in FIG. 9 (3), the danger side muting sensors A2, B2 are tuned ON, whereby the muting state is maintained. When the workpiece W further advances, the detection area of the multi-axis photoelectric sensor is returned to a state where it is not shielded, followed by turning OFF the muting sensor A2. In this example, the turn ON/OFF state is confirmed as described above, and the muting is ended when a state is reached where only the muting sensor B2 is in the ON-state.
Patent Document 1 listed below also describes a technique similar to that illustrated in FIGS. 9 (1) to (4), in which the muting sensor pairs are disposed, respectively, on the safety side and danger side, and muting is performed based on a status change of the detection signals from the respective sensors. In the muting processing described in Patent Document 1, the muting is started when the two safety side muting sensors are turned ON, followed by a state where the danger side muting sensors are turned ON, and the muting is ended when all the photoelectric sensors are turned OFF.